The proper functioning of the adult nervous system requires not only that neurons develop and interconnect properly during the embryonic period, but also that they retain some degree of plasticity in postembryonic life. As the nervous system matures, existing neurons may be modified to participate in new behavior. The primary objectives of this proposal are to understand the extent to which existing neurons can be modified structurally and functionally during postembryonic life, how these modifications are related to behavioral change, and how this plasticity is induced and regulated. Answers to these important questions require a simple model system in which it is possible to identify individual neurons and follow them as they are modified postembryonically. The experiments in this proposal concentrate upon the neural reorganization that accompanies insect metamorphosis, specifically upon the circuitry controlling the larval and adult legs of the moth, Manduca sexta. Although many of the same neurons are retained, the function of the legs is quite different in two stages. These retained neurons must delete synaptic connections important for larval behavior and form new interconnections appropriate for driving the adult legs. In this simple model system, intracellular recording techniques will be used to identify individual motorneurons, sensory neurons, and interneurons and to inject persistent marker molecules which will be used to follow these cells through metamorphosis. Specific aims are to determine which neurons are modified structurally and functionally, and to relate these modifications to changes in behavior. Furthermore, just as steroid hormones are known to influence the development of vertebrate nervous systems, steroid and steroid-like hormones are important regulators of insect metamorphosis. Precise hormonal manipulations, including the introduction of steroids into individual neurons, will be used in the future to pursue the mechanisms responsible for the induction of neural plasticity. These experiments will result in a better understanding of the cellular mechanisms underlying the development of normal neural function, and provide a model system which may be useful in the future for investigating the molecular mechanisms involved in neuronal differentiation.